The present invention relates to a focusing optical system for semiconductor lasers and, more particularly, to a focusing optical system for a semiconductor laser, which enables a light beam emitted from the semiconductor laser to be focused with so high efficiency that energy can be well obtained.
A semiconductor laser, because of being small, inexpensive and suitable for mass-production, has been used as a light source for laser printers and optical disk pickups, and has been utilized as an energy source of high power as well. However, a light source built up of a high-power semiconductor laser obtained by the integration of a number of minuscule light sources is in linear configuration, with the divergent angle in the direction vertical to the longitudinal direction being much larger than that in the longitudinal direction, so that no efficient focusing can be achieved with the use of a rotation symmetric optical system. In order to achieve efficient focusing, therefore, it is known that a rotation asymmetric optical system is used to focus a light beam on a spot with the longitudinal diameter substantially equal to the vertical diameter. An optical system having such action is known from JP-A-61-254915, but it says nothing about what problems arise practically in connection with aberration correction, back focus length, etc. Some other rotation asymmetric optical systems are disclosed in JP-A-4-55016 and JP-A-4-153618.
Light emitted from a linear laser emission portion is much larger in terms of the divergent angle in the direction vertical to the longitudinal direction rather than in the longitudinal direction. In this regard, JP-A-4-55016 teaches the use of a simple combination of two cylindrical lenses, whereby the laser beam diameters in the vertical and horizontal directions are made equal to each other. This means that the allocation of power to the longitudinal and vertical directions is calculated at such magnification power that provides a demagnifying system in the longitudinal direction and a magnifying system in the direction vertical thereto. At a high numerical aperture, however, this alone offers some practical problems, because any efficient focusing is not achievable due to a divergent off-axial light beam. JP-A-4-153618 discloses a lens system designed with the focusing efficiency in mind. However, no care is taken of the magnification of the enlarging system, because the line width of the linear light source is infinitely small. Nor is any care taken of back focus length; in other words, the mounting of peripheral devices offers some problems at a short back focus length.
Recently, there is also an available stratified light source with the line width being not taken as infinitesimal, in which case the magnification power in the vertical direction must be regulated. With this, focusing must also be done with suitable aberrations so as not to allow the image vertical to the longitudinal direction to be in a stratified form.